Setting or updating a profile image
Some email clients will show a profile image next to your emails.
This profile image cannot be set in the email itself, but there are a number of locations where email clients generally pull that picture from.
If you have a Google profile associated with the email address, you can set or update your profile picture on your about me screen. If not, you can create an account with the email address you're sending from.
Gravatar is a free service that associates an image with your email address. Sign up or access your existing account to manage the image.
If you've updated both of these and you're not seeing a profile image, make sure you have set up SPF and DKIM on your domain. Verifying your domain is the easiest way to do this.